The present invention relates to a method for converting, on an economically feasible basis, coals and other solid organic materials, such as organic waste or biomass, into a gas having an equivalent energy content suitable for use as a fuel for boilers and other energy conversion systems. More particularly, the present invention is directed to an improved method for converting, without the need for a large-scale, mechanically complex apparatus, solid fossil fuels and other organic materials to a relatively low-Btu gas which may then be economically burned on a relatively small scale to produce usable energy. The solid fossil fuels may be anthracite or other coals, or peat. In addition, other materials with a high organic component such as biomass, plant wastes, paper, and plastics can be used as feed for the gasifier. These other materials can be used as a feed either alone or in some combination with coal.
The conversion of solid organic materials into a suitable fuel gas is accomplished by exposing the solid organic materials to an intense field of radiation in an environment comprising an appropriate mixture of oxygen, and water. The fuel gas that results is a mixture of carbon monoxide, hydrogen, and small quantities of other gases such as carbon dioxide, with the amount of each product determined by the temperature of the reactor and the proportion of nitrogen, oxygen, and water in the initial reactor environment.
The fuel gas produced by the method of the present invention will have a relatively low energy value when compared to natural gas and other heating gases. However, the fuel gas can be utilized without further processing in a boiler, a gas turbine, or other energy conversion device, to produce useful energy, if the device is appropriately designed for such a low-energy fuel gas. The total equivalent energy value of the fuel gas will be between 180 to 220 Btu per cubic foot if air (approximately 20% oxygen and approximately 80% nitrogen) is supplied to the reactor, and may be as high as 350 Btu per cubic foot if 100% oxygen is supplied to the reactor. Mixtures of oxygen and nitrogen of intermediate proportions will produce fuel gases with intermediate energy values. In comparison, natural gas has an energy content of approximately 950 Btu per cubic foot.
The need to both utilize solid fossil fuels more efficiently and to use renewable fuels more extensively is becoming urgent. It is also essential that these fuels be used in a manner which minimizes any potential contribution to environmental pollution. As supplies of oil and gas are depleted and the future availability of the world's oil supply becomes increasingly questionable, the use of coal and other solid fuel reserves will become increasingly important. Solid fuels, however, often contain larger amounts of impurities than liquid or gaseous fuels. The combustion of these impurities along with the solid fuels results in the release of more pollutants to the environment than would occur due to the combustion of alternative liquid or gaseous fossil fuels.
The use of solid fuels such as coal remains an attractive alternative because of the large reserves of coal available and the relatively low cost of coal. However, the use of coal presents obvious disadvantages. Among these are the greater degree of pollution resulting from such use and the necessary limitation to stationary applications only. These disadvantages to the use of coal have spurred interest in the development of technology to convert solid fuel sources such as coal, tar sands, and oil shale to liquid and gaseous fuels.
The basic processes which underlie the conversion of solid fuels into gaseous and liquid fuels were known well back into the nineteenth century. During World War II, Germany relied heavily on gasification plants--plants for the conversion of solid and liquid fuel sources into gaseous fuels--for much of its energy needs. Today, a plant based on fundamentally similar processes operates in South Africa. Recently, large-scale gasification plants have been constructed in the U.S. that operate to produce a wide variety of materials.
The fundamental chemical and physical processes that form the basis for the conversion of solid fuel sources into liquid or gaseous fuels, although complex, have been studied extensively and are reasonably well-characterized. These processes and their application in commercial-scale projects has been reviewed in publications such as An Introduction to Coal Technology, by N. Berkowitz (Academic Press, New York, 1979), which is incorporated here by reference as is set forth in the text.
Although coal reserves are plentiful and relatively low in cost in comparison to other fossil fuels, the costs of converting solid fuel sources into gaseous fuel products are high. Consequently, fuel gas produced synthetically has a cost per Btu of energy that is greater than the cost of other naturally-occurring fossil fuels such as natural gas and oil.
The current state of development of gasification technology is directed primarily toward producing pipeline-quality fuel gases, such as synthetic natural gas (SNG), that have a Btu content comparable to naturally-occurring gaseous fuels (approximately 950 Btu per cubic foot). Economies of scale dictate the size at which these plants must operate to produce fuel gas at costs that are competitive with the costs of naturally-occurring liquid and gaseous fossil fuels. In some cases, the costs of gasification facilities and their operation are subsidized by governments. This is often a necessity due to the large capital expenditures required for plant construction. Without such subsidies, in many cases, the economies of scale necessary to render synthetic gas production competitive with fossil fuels could not be attained. Environmental concerns over the construction and operation of such large plants also adds significantly to these costs.
A number of processes have been developed which attempt to address some of the concerns expressed above concerning coal gasification. For example, U.S. Pat. No. 4,137,052 to du Pont discloses an apparatus and system for producing a coal gas of medium energy content (300-500 Btu per cubic foot) from a mixture of coal, air and water. The du Pont apparatus and system uses a series of retort reactors to produce a fuel ga mixture that is relatively high in methane and other hydrocarbons. Reaction temperatures in the retorts are attained by burning a portion of the product fuel gas stream in burners situated within an annular wall structure.
U.S. Pat. No. 4,177,120 to Zenty discloses a process for gasification of carbonaceous material in which carbon dioxide is converted to carbon monoxide by subjecting the carbon dioxide to solar radiation in an oxygen-free environment. The primary use of solar radiation in this process is to achieve a photodissociation of the carbon dioxide which results in reduction of the carbon dioxide to carbon monoxide by carbon present in the reactor. When the carbon dioxide is irradiated in the presence of other carbonaceous materials such as coal, the resulting product gas mixtures contains not only carbon monoxide but also methane and other hydrocarbons. The fuel gas produced by this process has a relatively high Btu content.
U.S. Pat. No. 4,229,184 to Gregg discloses an apparatus and method for gasifying coal and other carbonaceous material by using focused solar radiation. In this process, solar radiation is collected and focused by a set of primary and secondary mirrors into a reaction chamber, mounted on top of a tower, onto a moving or fluidized bed of carbonaceous material such as coal. The primary purpose of irradiation of the coal bed is to raise the temperature of the reactor so that pyrolysis of the coal occurs. In pyrolysis, volatiles are driven out of the coal, leaving essentially solid carbon or char. The resulting char then participates in subsequent reactions within the chamber, along with the volatiles driven off during pyrolysis. Next, steam is passed through the bed of carbonaceous material in the reaction chamber, producing a fuel gas mixture containing primarily carbon monoxide and hydrogen.
U.S. Pat. No. 4,415,339 to Aiman et al. discloses a gasification reactor in which coal or other carbonaceous material is converted into a hydrocarbon-free gas mixture through recycling of gases produced through pyrolysis of the carbonaceous material. In the Aiman reactor, solar radiation acts to pyrolyze the coal feed, creating char and pyrolysis gases. The pyrolysis gases are withdrawn from the reaction zone within the reactor, mixed with reactive gases such as steam, carbon dioxide, hydrogen, and methane and passed back into the reaction chamber where hydrocarbons present in the gas mixture are steam reformed into primarily carbon monoxide and hydrogen gases. Solar energy is used to provide the energy necessary for the reformation and gasification processes.